The recuperator at which the invention is aimed is of the kind which comprises a heat exchanger positioned inside a connecting pipe designed to be connected to the exhaust pipe so as to perform counter-current preheating of the combustion air by part of the flue gases, the exchanger comprising:                an outbound section for directing the air that is to be preheated toward a ferrule positioned at the end of the recuperator at the flue gas inlet end,        and a return section opening toward a line supplying the burner with air, the ferrule defining a path for reversing the direction of flow of the combustion air and directing it toward the return section,the assembly being designed so that part of the flue gases is entrained by and mixed with the combustion air so as to reduce the level of nitrogen oxides in the products of combustion.        
A heat recuperator of this kind is disclosed in FR-A-2 780 770. In general, the radiant tubes with burner are equipped with a system for recuperating heat from the flue gases, this system being of the finned exchanger type.
WO 2008/022722 also discloses a heat recuperator in which the outbound section is surrounded by the return section. The combustion air is heated by the flue gases only during the return path.
Recuperators proposed hitherto need to be improved in order:                to improve their thermal performance and the transfer of heat between the flue gases and the combustion air,        to increase the level of recirculation of flue gases in the combustion air so as to reduce emissions of nitrogen oxides and improve the temperature uniformity of the radiant tube,        to limit pressure drops so as to reduce the required pressure for the combustion air and the extraction needed to carry away the flue gases,        to gain control over pressure drops,        to make the cold ignition of the burner more reliable,        and to reduce the cost of manufacture.        